[GHC] #9270: GHC HEAD accepts a manual instance decl but not equivalent TH decl

[GHC]

#9270: GHC HEAD accepts a manual instance decl but not equivalent TH decl
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        Reporter:  Feuerbach         |            Owner:
            Type:  bug               |           Status:  closed
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.9
      Resolution:  invalid           |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Changes (by goldfire):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 The code in `traverse-with-class` is incorrect and should lead to exactly
 this error. It calls `classP` on a type variable `c`. `classP` is meant to
 make a class-predicate, and should be passed the name of a class, not a
 type variable. The fact that this worked previously was in error.

 To be fair, using a type variable in a constraint was not possible before
 GHC 7.9, so the author of `traverse-with-class` didn't really have another
 choice. Now that TH supports a full variety of constraints, the code can
 be rewritten to use `AppT` and `VarT` instead of `classP`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9270#comment:1>
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